This invention relates to the deposition of layers onto substrates, and, more particularly, to chemical vapor deposition of silicon carbide.
An extensive technology of semiconductor devices has been developed based upon the properties of single crystal silicon and other similar materials which may be doped, heat treated, and otherwise processed to produce adjacent layers and regions of varying electronic characteristics. The use of devices produced by silicon technology is generally limited to operation at ambient or, at most, moderately elevated temperatures and in non-corrosive, inert atmospheres. The temperature limitation is a consequence of the rapid diffusion of dopants or impurity species in the silicon, which in turn can substantially alter the character of the fabricated semiconductor device. The limitation to relatively inert environments results from the high chemical reactivity of silicon in many corrosive environments, which also can alter the character of the fabricated device. Silicon devices are also limited as to power level, frequency, and radiation tolerance by the materials used therein.
For some applications, the temperature, environmental, and other use limitations on silicon devices may be overcome by the use of proper cooling and packaging techniques. In other applications, these limitations have prevented the use of silicon for integrated circuit technology. For example, in many spacecraft and aircraft applications, elevated temperatures are encountered, and it is not always possible to insure that adequate cooling will be provided. In high power applications, internal thermal transients in devices otherwise operating at ambient temperature can rapidly destroy the operability of the device unless extensive cooling is provided. Such cooling requires that the device be larger in size that might otherwise be necessary, in part defeating the purpose of the integrated circuit technology.
There has therefore been an ongoing, but as yet not fully successful, search over a period of twenty years to identify and develop a semiconductor technology based in other materials. Such a technology would desirably allow the fabrication of devices for use at higher temperatures such as, for example, the range of at least about 400 C. to 600 C., and in applications not amenable to the use of silicon. Because corrosive effects can be greatly accelerated at elevated temperatures and pressures, any such materials and devices must also exhibit excellent corrosion resistance at the elevated use temperatures and over a range of pressures from vacuum to many atmospheres. Some generally desirable characteristics of such materials have been identified, including large band gap, good electrical conductivity, high breakdown electric field, low dielectric constant, ability to be doped to produce regions of varying electronic characteristics, a high melting temperature, good strength at operating temperatures, resistance to diffusion by undesired foreign atoms, good thermal conductivity, thermal stability, chemical inertness, and the ability to form ohmic external contacts.
Silicon carbide, particularly in its beta-phase form having a zincblende cubic crystallographic structure, has been identified as a candidate material meeting the indicated requirements. Silicon carbide has a high melting point, good strength, good resistance to radiation damage, and good corrosion resistance in many environments. Silicon carbide has a high breakdown voltage, a relatively large band gap, low dielectric constant, and a thermal conductivity of more than three times that of silicon at ambient temperature. Silicon carbide is also resistant to the diffusion of impurity species. Silicon carbide may be processed by several techniques similar to those used in silicon device technology, and in many instances silicon carbide devices may be substituted at moderate and low temperatures for silicon devices. Silicon carbide semiconductor device technology therefore offers the opportunity for supplementing, and in some instances replacing, conventional silicon device technology.
Silicon carbide may be formed or deposited by many techniques, one of which is chemical vapor deposition (CVD). In CVD, the species to be deposited are initially provided in a molecularly combined form. These molecules are selected to have a sufficiently high vapor pressure that they can be evaporated and transported in the vapor phase to a heated substrate. At the substrate, the molecules decompose by pyrolysis, depositing the species of interest on the substrate. Chemical vapor deposition is a particularly desirable fabrication approach, as it permits the controlled growth of undoped and doped layers and structures of a variety of forms.
Thus, in one common approach for depositing silicon carbide onto a substrate, silane (SiH.sub.4) is selected as the source of silicon, and n-hexane (C.sub.6 H.sub.14) or methane (CH.sub.4) is selected as the source of carbon. A mixture of these two gases in a hydrogen carrier gas is passed over a substrate maintained at a temperature of about 1400 C. Silicon carbide is deposited upon the substrate as the silicon-containing species and the carbon-containing species pyrolyze at the substrate.
This approach is operable in depositing silicon carbide, but cannot be controlled with sufficient precision to ensure that beta silicon carbide of stoichiometric composition, free of excess silicon or carbon, can be reproducibly deposited at different times of the same deposition run, or on different deposition runs. The principal difficulty is that absolutely reproducible flows of the source gases cannot be provided with existing gas flow control equipment.
Several approaches have been tried in the search for a solution to this problem. The very finest gas flow control equipment has been used in the CVD systems, but small disparities remain between successive deposited films. Sources in which the silicon and carbon are provided in a single molecule have been tried, but the deposited silicon carbide has been either silicon rich or carbon rich--it has proved impossible to obtain reproducibly precise stoichiometric compositions.
Accordingly, there exists a need for an improved process for depositing beta silicon carbide by chemical vapor deposition. Such a process should function by the basic CVD approach, but should result in stoichiometric, unpolytyped beta silicon carbide. The present invention fulfills this need, and further provides related advantages.